我正在尝试在我的应用程序中使用 AlertDialog，但它需要提供输入。因此，我被迫创建一个接口(interface)并重写自定义 DialogFragment 类中的 onAttach() 方法。一旦我正确设置了 Dialog 类和 MainActivity 中的所有内容，我尝试运行我的应用程序，它给了我一个运行时异常，其中涉及空指针。我不确定为什么会这样，我需要帮助，请帮忙。下面您将缠绕我的接口(interface)方法的重写、调用我的 .show() 方法的位置、我的 DialogFragment 类的代码以及最后的错误日志。
public class MainActivity extends FragmentActivity implements CDia_exp.NDListener
{
//Integers and Strings for performing calculations
int a;
int b;
static int deci_cnt;
int cnt;
int temp;
Double exp_x;
Double exp;
Double [] num_trk;
String [] op_trk;
String num_hold;
String op_hold;
String del_hold;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//....Lots of other code here.......
//Exp_x Button
opSet[6].setOnClickListener(new View.OnClickListener()
{
public void onClick(View v)
{
if(num_hold != "")
{
cDia_exp.show(getSupportFragmentManager(), "exp_x");
del_hold = tViews[0].getText().toString();
for(int x = num_hold.length() - 1; x >= 0; x--)
{
del_hold = del_hold.substring(0, del_hold.length() - 1);
}
num_hold = exp.toString();
tViews[0].setText(del_hold + num_hold);
return;
}
else
{
return;
}
}
});
}
//Objects and Overrides for calling foreign functions
CTrim cTrim = new CTrim();
CDia cDia = new CDia();
CDia_exp cDia_exp = new CDia_exp();
EditText exp_inp = (EditText) findViewById(R.id.exp_inp);
@Override
public void onDPClick(DialogFragment dialog)
{
exp = Double.parseDouble(num_hold);
exp_x = Double.parseDouble(exp_inp.getText().toString());
for(double x = exp_x; x > 0; x--)
{
exp *= exp;
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
}
我的自定义DialogFragment类
package com.example.musicalc;
import android.app.Activity;
import android.app.AlertDialog;
import android.app.Dialog;
import android.content.DialogInterface;
import android.os.Bundle;
import android.support.v4.app.DialogFragment;
import android.view.LayoutInflater;
public class CDia_exp extends DialogFragment
{
public interface NDListener
{
public void onDPClick(DialogFragment dialog);
}
NDListener expListener;
@Override
public void onAttach(Activity activity)
{
super.onAttach(activity);
try
{
expListener = (NDListener) activity;
}
catch(ClassCastException e)
{
throw new ClassCastException(activity.toString() + "does not implement NDListener");
}
}
@Override
public Dialog onCreateDialog(Bundle savedInstanceState)
{
AlertDialog.Builder aDia = new AlertDialog.Builder(getActivity());
LayoutInflater inflater = getActivity().getLayoutInflater();
aDia.setView(inflater.inflate(R.layout.calc_dia, null)).setPositiveButton(R.string.diaOk_b, new DialogInterface.OnClickListener()
{
@Override
public void onClick(DialogInterface dialog, int id)
{
expListener.onDPClick(CDia_exp.this);
}
}).setNegativeButton(R.string.cancel_b, new DialogInterface.OnClickListener()
{
@Override
public void onClick(DialogInterface dialog, int id)
{
getDialog().cancel();
}
});
return aDia.create();
}
}
现在是错误日志
07-20 14:23:43.599: E/Trace(24173): error opening trace file: No such file or directory (2)
07-20 14:23:43.622: D/AndroidRuntime(24173): Shutting down VM
07-20 14:23:43.622: W/dalvikvm(24173): threadid=1: thread exiting with uncaught exception (group=0x4137d2a0)
07-20 14:23:43.653: E/AndroidRuntime(24173): FATAL EXCEPTION: main
07-20 14:23:43.653: E/AndroidRuntime(24173): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.musicalc/com.example.musicalc.MainActivity}: java.lang.NullPointerException
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2060)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2174)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.ActivityThread.access$700(ActivityThread.java:141)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1267)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.os.Handler.dispatchMessage(Handler.java:99)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.os.Looper.loop(Looper.java:137)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.ActivityThread.main(ActivityThread.java:5059)
07-20 14:23:43.653: E/AndroidRuntime(24173): at java.lang.reflect.Method.invokeNative(Native Method)
07-20 14:23:43.653: E/AndroidRuntime(24173): at java.lang.reflect.Method.invoke(Method.java:511)
07-20 14:23:43.653: E/AndroidRuntime(24173): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
07-20 14:23:43.653: E/AndroidRuntime(24173): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
07-20 14:23:43.653: E/AndroidRuntime(24173): at dalvik.system.NativeStart.main(Native Method)
07-20 14:23:43.653: E/AndroidRuntime(24173): Caused by: java.lang.NullPointerException
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.Activity.findViewById(Activity.java:1851)
07-20 14:23:43.653: E/AndroidRuntime(24173): at com.example.musicalc.MainActivity.<init>(MainActivity.java:43)
07-20 14:23:43.653: E/AndroidRuntime(24173): at java.lang.Class.newInstanceImpl(Native Method)
07-20 14:23:43.653: E/AndroidRuntime(24173): at java.lang.Class.newInstance(Class.java:1319)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
07-20 14:23:43.653: E/AndroidRuntime(24173): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2051)
07-20 14:23:43.653: E/AndroidRuntime(24173): ... 11 more
07-20 14:25:56.817: E/Trace(24308): error opening trace file: No such file or directory (2)
07-20 14:25:56.934: D/AndroidRuntime(24308): Shutting down VM
07-20 14:25:56.934: W/dalvikvm(24308): threadid=1: thread exiting with uncaught exception (group=0x4137d2a0)
07-20 14:25:57.013: E/AndroidRuntime(24308): FATAL EXCEPTION: main
07-20 14:25:57.013: E/AndroidRuntime(24308): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.musicalc/com.example.musicalc.MainActivity}: java.lang.NullPointerException
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2060)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2174)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.ActivityThread.access$700(ActivityThread.java:141)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1267)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.os.Handler.dispatchMessage(Handler.java:99)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.os.Looper.loop(Looper.java:137)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.ActivityThread.main(ActivityThread.java:5059)
07-20 14:25:57.013: E/AndroidRuntime(24308): at java.lang.reflect.Method.invokeNative(Native Method)
07-20 14:25:57.013: E/AndroidRuntime(24308): at java.lang.reflect.Method.invoke(Method.java:511)
07-20 14:25:57.013: E/AndroidRuntime(24308): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:792)
07-20 14:25:57.013: E/AndroidRuntime(24308): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:555)
07-20 14:25:57.013: E/AndroidRuntime(24308): at dalvik.system.NativeStart.main(Native Method)
07-20 14:25:57.013: E/AndroidRuntime(24308): Caused by: java.lang.NullPointerException
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.Activity.findViewById(Activity.java:1851)
07-20 14:25:57.013: E/AndroidRuntime(24308): at com.example.musicalc.MainActivity.<init>(MainActivity.java:803)
07-20 14:25:57.013: E/AndroidRuntime(24308): at java.lang.Class.newInstanceImpl(Native Method)
07-20 14:25:57.013: E/AndroidRuntime(24308): at java.lang.Class.newInstance(Class.java:1319)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
07-20 14:25:57.013: E/AndroidRuntime(24308): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2051)
07-20 14:25:57.013: E/AndroidRuntime(24308): ... 11 more
最佳答案
感谢一些实验和 stackoverflow 上的一些研究，我找到了答案。
问题是编译器无法定位 View ，因为它位于与我的 Activity_main.xml 文件不同的 xml 文件中，或者因为我在与我的 MainActiviy.java 不同的类中创建对话框。
为了解决这个问题，我在 onCreate 方法之外的 MainActivity.java 类中创建了对话框。此外，我还创建了一个布局充气器来充气 MainActivity 中 View 中的布局，这很关键，因为 LayoutInflater 提供了一种直接将 EditText 调用到代码中的方法。
声明我的 EditText 时修复空指针的代码如下:
LayoutInflater inflater = LayoutInflater.from(this);
//Inflater "inflates" layout into View as a value
final View infV = inflater.inflate(R.layout.calc_dia, null);
//Use layout value inflated in View "infV" to find EditText
final EditText eDTemp = (EditText) infV.findViewById(R.id.expInp);
***对话框的完整代码如下****:
public void showCD()
{
LayoutInflater inflater = LayoutInflater.from(this);
final View infV = inflater.inflate(R.layout.calc_dia, null);
final EditText eDTemp = (EditText)infV.findViewById(R.id.expInp);
TextView tView0 = (TextView)findViewById(R.id.calcView);
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setView(infV).setPositiveButton("Ok", new
DialogInterface.OnClickListener()
{
@Override
public void onClick(DialogInterface dialog, int which)
{
}}).setNegativeButton("Cancel", new DialogInterface.OnClickListener()
{
@Override
public void onClick(DialogInterface dialog, int which)
{
dialog.cancel();
}
});
AlertDialog alert = builder.create();
builder.show();
}
当您准备好显示对话框时，只需调用包含该对话框的函数即可。
关于java - Dialog Fragment 在 MainActivity 中实现后抛出空指针异常，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/24853752/